In the High-Ardenne Slate Belt (Belgium), pre-existing layer-perpendicular quartz veins in psammite layers acted as mechanical boundaries due to the difference in competence between vein quartz and psammite. This caused the formation of mullions during layer-parallel shortening. In this paper, the process of mullion formation is modelled using finite-element techniques. A parameter sensitivity analysis of the mullion model demonstrates that the stress exponent of psammite, the total horizontal shortening of the psammite layers, the initial aspect ratio of the psammite segments between the veins and the competence contrast between psammite and vein quartz are the controlling parameters for the shape of the mullions. Our results suggest that the morphology of the mullions can be used to constrain the rheology of psammite deforming in the middle crust. Moreover, the parameter sensitivity analysis illustrates the range of layer-parallel shortening/extension structures associated with layer-perpendicular quartz veins that can be expected in nature (e.g. dogbones, inverted mullions) and creates future perspective for the use of the model as a paleorheological gauge for these structures. (C) 2004 Elsevier Ltd. All rights reserved.